even, regular, uniform all alike, to arrange
亝 centers on uniformity and alignment: things that are even, regular, or arranged to be alike. The idea of arranging something to make it uniform connects the given meanings.
亝 is an ancient variant of 齊 (even, uniform). The modern form 亝 is a simplified representation, but the exact historical development of the 厽 component is uncertain.
The top 厽 looks like three stacked private parts (ム repeated), and the bottom 二 is two. Imagine arranging three private items into two equal rows to make everything even and uniform.
For セイ, picture arranging everything evenly on a say (セイ) — like a balanced scale where you say 'say' as you place the last item to make it uniform.